Incomplete immunization with the quadrivalent human papillomavirus (HPV) vaccine still appears to offer some protection against disease, researchers reported.

Action Points

Note that this Swedish cohort study suggested that fewer than the recommended three shots with the quadrivalent HPV vaccine was still associated with reduced risk of disease.

Be aware that the outcome studied, condyloma incidence, may not be a perfect surrogate for cervical cancer risk.

Incomplete immunization with the quadrivalent human papillomavirus (HPV) vaccine still appears to offer some protection against disease, researchers reported.

In a population-based study in Sweden, the full series of three shots offered 82% protection against condyloma, or genital warts, compared with no vaccination, according to Lisen Arnheim-Dahlström, PhD, of the Karolinska Institute in Stockholm, and colleagues.

The vaccine is intended to prevent infection by the HPV types that lead to cervical cancer and thereby prevent malignancies. But condyloma, a benign tumor, represents "the earliest measurable preventable disease outcome," the researchers noted.

However, more study will be needed to see the effect of less-than-complete immunization on the risk of cervical cancer, Arnheim-Dahlström and colleagues concluded.

"It is reassuring to know that those girls who received at least two doses will likely get a substantial protection benefit," Bonnez told MedPage Today by email.

Starting in 2007, Arnheim-Dahlström and colleagues noted, Sweden had a partly subsidized vaccination program for girls, ages 13 to 17, that reached about 25% of the target population.

In 2012, that was replaced by a free school-based vaccination program for girls, ages 10 through 12, with a catch-up program (also free) for girls 13 to 18.

For the study, they used registry data, including medical records and prescription information, to look at HPV vaccination and condyloma incidence in all Swedish girls and young women ages 10 through 24 from Jan. 1, 2006 through Dec. 31, 2010.

All told, 1,045,165 people in the cohort contributed 3,995,631 person-years of follow-up, with 20,383 incident cases of condyloma, including 322 cases after at least one dose of the vaccine.

For girls who were 10 through 16 at their first vaccination, analysis showed getting three doses of vaccine was associated with an incidence rate ratio (IRR) of 0.18 for condyloma, compared with no vaccination. That corresponded to prevention of 459 cases per 100,000 person-years.

Also, receipt of two doses was associated with an IRR 0.29, which corresponds to prevention of 400 cases per 100,000 person-years compared with no vaccination.

Finally, getting one dose was associated with an IRR of 0.31, which corresponds to preventing 384 cases per 100,000 person-years compared with no vaccination.

The analysis also showed that getting the third dose -- versus stopping at two -- prevented 59 cases of condyloma per 100,000 person-years, Arnheim-Dahlström and colleagues reported.

The researchers cautioned that the registry data might have underestimated the number of cases of condyloma, although under-reporting should not be different with regard to vaccination status. And, they noted, it does not account for other HPV disease outcomes.

Rochester's Bonnez said it's still too early to suggest changing the vaccination schedule, especially since the effect against cervical cancer has yet to be established.

"The demonstration in a population-based setting that vaccination will be effective against HPV-associated cancers will require a few more years," he said, although experts have "little doubt" that it will eventually be seen.

It will also be important, he said, to know the magnitude of the effect, not only for cervical cancer, which has established prevention programs, but also for HPV-associated malignancies that do not, including cancers of the vulva, vagina, penis, anus, and oropharynx.

"More remains to be established before immunization guidelines can be changed drastically," Bonnez argued.

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.